Technical Field
The present invention relates to a laser sintering powder useful as a raw material powder in selective laser sintering and a shaped article obtained using the same, and more specifically relates to a laser sintering powder which has low porosity and is excellent in strength and elongation. Lastly, the present invention relates to a method of making a sinterable article via selective laser sintering.
Although laser sintering is an emerging field, it has many features that make it attractive, such as forming parts or articles that do not require a mold, employing just as much raw material as the parts or article need, no further tooling requirements, and using just enough energy to form the part or article and no long heating and cooling steps.
Background Art
Techniques of design and/or plan of products or components with computer software such as CAD, CAM and CAE have become popular in various industrial fields for automobiles, airplanes, buildings, home appliances, toys and convenience goods. A method for producing a physical model which is materialized from a three-dimensional model designed with CAD is called rapid prototyping system, rapid manufacturing system or the like.
The rapid prototyping systems have a delivery aperture for the laser sintering powder of a maximum opening of 150 microns. Therefore the maximum particle size of the laser sintering powder is typically no larger than 150 microns to allow it to pass through the opening. Other systems may have larger delivery openings, but generally larger particle sizes mean less precise definition of the laser sintered article. The rapid prototyping systems include selective laser sintering in which a thin layer of resin powder is selectively irradiated with a laser beam in accordance with slice data obtained by converting CAD data or the like of a three-dimensional shaped article so as to allow the powder to selectively sinter and adhere to each other. Then, another thin layer of resin powder is formed on the first formed layer. This same operation is repeated so as to perform sequential layer buildup to obtain a shaped article.
In selective laser sintering, shaping is performed by heating a layer of a resin powder (sometimes referred to as Selective Mask Sintering), or a portion of a layer, which has been fed to a platform upon which an article is to be formed. The entire layer mass is heated to about the softening point of the resin powder, in order to shorten the time required for shaping, and then the thin layer mass is selectively irradiated using a laser beam thereby allowing each layer to fuse with the other.
A typical example of the resin powder currently used in laser sintering is polyamide resin. Polyamide resin is a semi-crystalline resin, and has a softening point corresponding to the melting point thereof. Since polyamide resin is high in laser beam absorption, it easily reaches its melting point or higher temperature by irradiation with a laser beam and fluidizes so as to fuse each layer with each other. However, shaped articles obtained by laser sintering are generally in a porous state, and thus require a sealing treatment by vacuum impregnation to acquire hermeticity. Such post-treatment scaling can involve impregnating water-soluble polyurethane in the shaped article. The porous nature of the sintered polyamide articles is understood to result from the relatively high water absorption of polyamide. Upon irradiation and heating the absorbed water vaporizes to create voids and pores.
A laser sintering powder composition which is easily fusible and does not require such a post-treatment (especially of impregnating water-soluble urethane) has been desired.
On the other hand, styrene resin such as ABS (acrylonitrile-butadiene-styrene) resin is low in water absorption (demonstrating low porosity), and not only excellent in mechanical strength such as impact resistance but also excellent in secondary processability such as in coating and plating, and further provides transparent shaped articles. Thus, it is attractive as a raw material fir laser sintering processes. However these resins are rigid and yield hard, rigid sintered articles. They are not suitable for the fabrication of soft and/or elastic articles.
Styrene resin is a non-crystalline, glassy resin and has a softening point corresponding to its glass transition temperature. When a non-crystalline, glassy resin powder is used as a raw material for laser sintering, it is required that the bed or platform temperature is maintained around its glass transition temperature. Then, a laser beam is employed to radiatively heat the material to its glass transition temperature (Tg) or higher in order to cause sintering and fusion of the layers. When the temperature of a non-crystalline, glassy resin powder is elevated slightly over the glass transition temperature by irradiation with a laser beam, the resin is still too high in melt viscosity to flow and the whole resin powder does not come to uniformly melt. Thus, the shaped articles tend to be porous and low in density because of the low flow characteristics. On the other hand, use of a high-output laser makes it possible to raise the temperature of non-crystalline, glassy resins much higher than the glass transition temperature. In this case, it is difficult to control heating and as a result powders outside the laser-scanning region may be sintered, causing what is known as sintering swell, and impair dimensional accuracy. In this way materials may deteriorate.
Block copolymers having high flow characteristics have been taught by Handlin in U.S. Pat. No. 7,439,301. Those polymers were elastomeric styrenic block copolymers having linear or radial structures and high vinyl contents.
U.S. Publ. 2013/022502.0 to Flood et al discloses many different end uses for high flow block copolymers, including slush molding, comprising a very high melt flow hydrogenated triblock copolymer of styrene-ethylene butadiene-styrene.
U.S. Publ. 2012/0070665 to Bellomo et al describes a thermofusible composition for slush molding applications having finely divided particles of small size that is made into a skin, such as a dash board, for an automobile. The composition employs a selectively hydrogenated, controlled distribution block copolymer of styrene(ethylene-butylene/styrene)styrene block copolymer (S(EB/S)S). These thermofusible compositions require slush- or roto-molding fabrication techniques which themselves utilize expensive equipment and experience high thermal energy utilization.
U.S. Publ. No. 2011/0129682 to Kurata et al entitled Laser-Sinterable Powder and Shaped Article Thereof utilizes minor amounts of rubber reinforced styrene resins in laser sintering compositions. In particular, rigid, selectively hydrogenated acrylonitrile-butadiene-styrene-type resins or random, tapered styrenic block copolymers were blended with semi-crystalline polyolefins to make laser sintering compounds, The resulting articles are expected to be rigid.
Thus, there remains a need to expand the utility of relatively low energy laser sintering processes. Further, as an alternative to compositions comprising rigid materials such as ABS and polyamide, it is desirable to make laser sintering compositions comprising soft, elastic materials. in this way, soft or elastic articles can he made using low energy laser sintering processes.